1. Field of the Invention
The present invention relates to electrode coatings and, more particularly, to the use of electrode coatings in electrolytic cells for sodium chlorate production and its method of preparation.
2. Description of the Related Art
An electrolytic cell is an electrochemical device that may be used to overcome a positive free energy and force a chemical reaction in the desired direction. For example, Stillman, in U.S. Pat. No. 4,790,923, and Silveri, in U.S. Pat. No. 5,885,426, describe an electrolytic cell for producing a halogen.
Other uses for an electrolytic cell include, for example, the electrolysis of an alkali halide solution to produce an alkali metal halate. In particular, sodium chloride (NaCl) solution may be electrolyzed to produce sodium chlorate (NaClO3) according to the general reaction:
NaCl+3H2Oxe2x86x92NaClO3+3H2xe2x80x83xe2x80x83(1) 
One effort to create such an apparatus has been described by de Nora et al., in U.S. Pat. No. 4,046,653, to produce sodium chlorate.
The design of electrolytic cells depends on several factors including, for example, construction and operating costs, desired product, electrical, chemical and transport properties, electrode materials, shapes and surface properties, electrolyte pH and temperature, competing undesirable reactions and undesirable by-products. Some efforts have focused on developing electrode coatings. For example, Beer et al., in U.S. Pat. Nos. 3,751,296, 3,864,163 and 4,528,084 teach of an electrode coating and method of preparation thereof. Also, Chisholm, in U.S. Pat. No. 3,770,613, Franks et al., in U.S. Pat. No. 3,875,043, Ohe et al., in U.S. Pat. No. 4,626,334, Cairns et al., in U.S. Pat. No. 5,334,293, Hodgson, in U.S. Pat. No. 6,123,816, Tenhover et al., in U.S. Pat. No. 4,705,610, and de Nora et al., in U.S. Pat. No. 4,146,438, disclose other electrodes. And, Alford et al., in U.S. Pat. No. 5,017,276, teach a metal electrode with a coating consisting essentially of a mixed oxide compound comprising ruthenium oxide with a compound of the general formula ABO4 and titanium oxide. In the ABO4 compound, A is a trivalent metal and B is antimony or tantalum.
Although these efforts may have produced some desirable electrode properties, other enhancements remain desirable.
In accordance with one embodiment, the invention provides an electrode comprising an electrically conductive substrate with an electrocatalytic coating covering at least a portion of a surface of the electrically conductive substrate. The electrocatalytic coating comprises an electrocatalytic agent comprising at least one of a precious metal, a precious metal oxide, a platinum group metal and a platinum group metal oxide, a stability enhancing agent comprising at least one of a precious metal, a precious metal oxide, a platinum group metal and a platinum group metal oxide, an oxygen suppressant agent comprising at least one of a Group V-A metal and a Group V-A metal oxide and an electroconductive binder comprising at least one of a valve metal and a valve metal oxide.
The invention also provides an electrolytic cell comprising an electrolyte in a cell compartment, an anode and a cathode immersed in the electrolyte and a power source for supplying a current to the anode and the cathode. The anode is coated with a mixture comprising ruthenium oxide, at least one of a platinum group metal and a platinum group metal oxide, antimony oxide and a valve metal oxide.
In another embodiment, the invention provides a method of producing sodium chlorate comprising supplying an electrolyte comprising sodium chloride to an electrolytic cell comprising electrodes with an electrocatalytic coating of a mixture comprising at least one of a metal and a metal oxide suppressing oxygen generation and at least one of a metal and a metal oxide enhancing coating stability. The method further comprises applying a current to the electrodes and recovering sodium chlorate from the electrolytic cell.
In yet another embodiment, the invention provides a method of coating an electrode comprising preparing a homogeneous mixture of salts of ruthenium, at least one of a precious metal and a platinum group metal, antimony and a valve metal, applying a layer of the homogeneous mixture on at least a portion of a surface of the electrode, drying the layer and heat treating the layer to form an electrocatalytic coating on the electrode.
In yet another embodiment, the invention provides an electrode comprising an electrocatalytic coating comprising about 10 to about 30 mole percent ruthenium oxide, about 0.1 to about 10 mole percent iridium oxide, about 0.5 to about 10 mole percent antimony oxide and titanium oxide.